1. Field of Invention
This invention pertains to rotating structures supported by magnetic bearings, and more particularly, a structure with auxiliary bearings constructed and arranged to permit the structure to withstand excessive radial and axial shocks.
2. Background of the Invention
High speed rotating structures such as various motors, generators, turbines, and so on, are provided with bearings interfacing between the stationary and rotating members. These bearings must be able to function reliably, without overheating.
Unlike other bearing support systems, magnetic bearings are limited in load capacity by the magnetic properties of the bearing materials. Typical magnetic journal bearing will support loads of 40 to 80 pounds per square inch of projected area, depending upon rotor and stator materials. In contrast, oil bearings are designed for loads of 100 to 250 pounds per square inch of projected area and can carry considerably higher loads for short periods of operation.
The magnetic bearing performs two functions, i.e., to levitate and statically support the rotor in a magnetic field and to provide stiffness and damping properties to control the position of the shaft when subjected to any dynamic forces. The static or support stiffness is generally 10 to 20 times the magnitude of the dynamic stiffness. The dynamic stiffness is further reduced at running speed to minimize the force transmission to the bearing pedestal from rotating unbalance, resulting in a relatively soft dynamic stiffness. The disadvantage of the soft suspension is the limited ability to absorb shock loading which could bottom out the bearing. There is a need, therefore, to provide a backup or catcher bearing to prevent contact of the rotor and stator in the event of high shock load. The backup bearing also provides protection of the magnetic bearing in the event of the bearing circuitry and/or winding failures and prevents contact of the magnetic bearing when the machine is not running.
The generally accepted procedure for incorporating this backup or catcher bearing into the magnetic bearing design is to mount the outer race of a rolling element bearing, typically a deep groove bearing into a rigid housing which holds the center of this backup bearing concentric to the rotating center line of the rotor. There is a machined clearance between the inner race of the backup bearing and the outer diameter of the rotor. This clearance is smaller then the air gap in the magnetic bearing. When the magnetic bearing is supporting the rotor, it holds the rotor within this clearance so that during normal operation, the rotor never touches the backup bearing. During failure of the magnetic bearing or during periods of high shock loading, the rotor is caught and/or supported by the back bearing. This concept is shown in FIG. 1, wherein a rotating structure 10 includes a housing 12 for supporting a rotor 14 by using magnetic bearing. The magnetic bearing consists of a bearing stator 16 mounted on the housing 12 and a bearing rotor 18 mounted on the rotor 14. In normal operation the magnetic bearing maintains the rotor at a nominal radial gap 22 between the bearing stator and the bearing rotor. In case of a failure of the magnetic rotor, or in response to a radial shock, the rotor may crash into the stator. In order to prevent such an occurrence, a conventional static bearing 24 is mounted on the housing as a backup bearing. For example the bearing 24 may be a deep groove roller bearing with an outer race mounted on the housing 12. The bearing 24 is sized so that there is a radial gap 26 between the bearing 24 and rotor 14 which is smaller than the gap 22 between the stator bearing and the rotor bearing.
FIGS. 2 and 2A show another rotating structure designed to take a thrust load. This structure 30 includes a housing 32 which supports a rotor 32 with an axial thrust runner 34. The structure is also provided with a radial magnetic bearing consisting of a journal stator 36 mounted in the housing and a journal rotor 38 mounted on the rotor, separated by a radial gap 40. A second magnetic bearing is also provided which includes thrust stators 42, 44 mounted in the housing 32. The thrust stator 42 and runner 34 are separated by an axial thrust gap 46. In order to prevent damage caused by the failure of a magnetic bearing, the structure is also provided with a pair of conventional backup bearings 50, 52 mounted securely in the housing 32 and separated by a preloading shim 54. The bearings are spaced by a radial gap 56 from the rotor, which gap is smaller than the journal gap 40, and by an axial gap 58 from a shoulder 60 formed on rotor 32. Axial gap 58 is smaller than the thrust gap 46.
However the magnetic bearing structures described above were limited in weight, diameter and speed because by ability of backup bearings to handle the rotor in case of a failure.